Systems and methods for controlling differential refrigerant pressure

ABSTRACT

Systems and methods are provided for controlling compressor systems to ensure sufficient pressure differentials to provide cooling. A compressor system includes a compressor, a suction pressure sensor at a suction of the compressor, a discharge pressure sensor, a condenser, an expansion device, a liquid line, a liquid line pressure sensor, an evaporator, a condenser blower and a controller. The method includes determining a pressure target based on an intermediate pressure within the compressor and a threshold cooling differential pressure value, determining a pressure ratio setpoint based on the pressure target and a liquid line pressure measured by the liquid line pressure sensor, controlling the condenser blower to operate based on the determined pressure ratio setpoint, determining a subcooling setpoint based on the pressure target and the liquid line pressure in the compressor system, and controlling the expansion device to operate based on the subcooling setpoint.

FIELD

This disclosure is directed to compressor systems and methods ofoperating compressor systems to control airflow and expansion valveopening to ensure a sufficient differential refrigerant pressure forcooling components of the compressor system.

BACKGROUND

Cooling systems for some compressors use a pressure differential withinthe compressor system to provide cooling to compressor system componentssuch as the compressor motor or an inverter. The pressure differentialmay need to be maintained at or above a threshold value to ensure thatthe cooling system functions properly. In some compressor systems, thedifferential pressure may vary depending on ambient temperature, watertemperature at an evaporator in chiller systems, and the operationalparameters of the compressor system.

SUMMARY

This disclosure is directed to compressor systems and methods ofoperating compressor systems to control airflow and expansion valveopening to ensure a sufficient differential refrigerant pressure forcooling components of the compressor system.

Refrigerant pressure differentials, such as a differential in pressurebetween the intermediate pressure within a compressor of the compressorsystem and a liquid line of the compressor system, may be used toprovide cooling to compressor system components such as motors,inverters, bearings, and the like. In some compressor systems,refrigerant pressure differentials must be maintained at or above athreshold value for proper cooling of those components.

By adjusting a pressure ratio target and a cooling setpoint, operationof the compressor system may be adjusted to ensure that a refrigerantpressure differential sufficient for cooling is maintained bycontrolling the airflow and the aperture of an expansion device.

In an embodiment, a method of controlling a compressor system includesdetermining a pressure target based on an intermediate pressure within acompressor of the compressor system and a threshold cooling differentialpressure value. The method also includes determining a pressure ratiosetpoint based on the pressure target and a liquid line pressure withina liquid line of the compressor system. The method includes operating acondenser blower at speed determined based on the determined pressureratio setpoint. The method further includes determining an subcoolingsetpoint based on the pressure target and the liquid line pressure inthe compressor system, and operating an expansion device based on thedetermined subcooling setpoint.

In an embodiment, the intermediate pressure is determined based on asuction pressure of the compressor and a discharge pressure of thecompressor.

In an embodiment, determining the pressure ratio setpoint includescomparing the pressure target and the liquid line pressure, when thepressure target exceeds the liquid line pressure, setting the pressureratio setpoint to a current pressure ratio multiplied by a ratio of thepressure target to the liquid line pressure, when the liquid linepressure meets or exceeds the pressure target, setting the pressureratio setpoint to the current pressure ratio.

In an embodiment, determining the speed comprises referencing amathematical model correlating pressure ratio and condenser blowerspeed. In an embodiment, the method further includes applying asmoothing function to control a rate of change of the pressure ratiosetpoint.

In an embodiment, the method further includes comparing the determinedpressure ratio setpoint to a pressure ratio limit and setting thepressure ratio setpoint to the pressure ratio limit when the determinedpressure ratio setpoint exceeds the pressure ratio limit.

In an embodiment, determining the subcooling setpoint includesdetermining a liquid pressure difference based on the pressure targetand the liquid line pressure, determining a subcooling offset based onthe liquid pressure difference, determining the subcooling setpointbased on the subcooling offset and a current subcooling value.

In an embodiment, determining the subcooling offset includes referencinga mathematical model correlating the subcooling offset with the liquidpressure difference. In an embodiment, the method further includesapplying a smoothing function to control a rate of change in thesubcooling setpoint.

In an embodiment, operating the expansion device based on the subcoolingsetpoint comprises setting an aperture size of the expansion devicebased on the subcooling setpoint.

In an embodiment, a compressor system includes a compressor, acondenser, an expansion device fluidly connected to the condenser by aliquid line, a liquid line pressure sensor at a liquid line of thecompressor system, an evaporator, a blower configured to drive anairflow over the condenser and a controller. The controller isconfigured to determine a pressure target based on an intermediatepressure within the compressor and a threshold cooling differentialpressure value, determine a pressure ratio setpoint based on thepressure target and a liquid line pressure measured by the liquid linepressure sensor, control the blower to operate at a speed determinedbased on the determined pressure ratio setpoint, determine a subcoolingsetpoint based on the pressure target and the liquid line pressure inthe compressor system, and control the expansion device to operate basedon the subcooling setpoint.

In an embodiment, the expansion device is a controllable electronicexpansion valve. In an embodiment, the processor is configured tocontrol the expansion device to operate based on the subcooling setpointby setting an aperture size for the controllable electronic expansionvalve.

In an embodiment, the processor is further configured to determine theintermediate pressure within the compressor based on a suction pressuremeasured by the suction pressure sensor and a discharge pressuremeasured by the discharge pressure sensor.

In an embodiment, the controller is configured to determine a pressureratio setpoint by comparing the pressure target and the liquid linepressure, when the pressure target exceeds the liquid line pressure,setting the pressure ratio setpoint to a current pressure ratiomultiplied by a ratio of the pressure target to the liquid linepressure, when the liquid line pressure meets or exceeds the pressuretarget, setting the pressure ratio setpoint to the current pressureratio.

In an embodiment, the controller is configured to determine a subcoolingsetpoint by determining a liquid pressure difference based on thepressure target and the liquid line pressure, determining a subcoolingoffset based on the liquid pressure difference, and determining thesubcooling setpoint based on the subcooling offset and a currentsubcooling value.

DRAWINGS

FIG. 1 shows a compressor system according to an embodiment.

FIG. 2 shows a flowchart of a method of controlling a compressor systemaccording to an embodiment.

FIG. 3 shows a flowchart of an example method for determining a pressureratio setpoint according to an embodiment.

FIG. 4 shows a flowchart of an example method for determining asubcooling setpoint according to an embodiment.

DETAILED DESCRIPTION

This disclosure is directed to compressor systems and methods ofoperating compressor systems to control airflow and expansion valveopening to ensure a sufficient differential refrigerant pressure forcooling components of the compressor system.

FIG. 1 shows a compressor system 100 according to an embodiment.Compressor system 100 includes compressor 102, condenser 104, expansiondevice 106 and evaporator 108, fluidly connected to one another.Compressor system 100 also includes controller 110. Compressor system100 further includes suction pressure sensor 112 at suction 114 ofcompressor 102, a discharge pressure sensor 116 at discharge 118 ofcompressor 102, and a liquid line sensor 120 located along a liquid line122 between the condenser 104 and expansion device 106 with respect tothe flow of working fluid through compressor system 100. A cooling line126 may run from liquid line 122 to compressor 102 and/or othercomponents of compressor system 100.

Compressor 102 is a compressor that compresses a working fluid of thecompressor system 100. Compressor 102 may be, for example, a centrifugalcompressor, or any other suitable type of compressor. Compressor 102 mayinclude a motor. In an embodiment, the motor of compressor 102 isconfigured to be cooled by the working fluid of compressor system 100.In an embodiment, the cooling of the motor of compressor 102 may beaffected by a pressure differential within the compressor system 102. Inan embodiment, cooling may be provided by introducing working fluid fromliquid line 122 via cooling line 126 into parts of compressor 102. Theworking fluid introduced into those parts of compressor 102 expands,absorbing heat and cooling the parts of compressor 102, then the workingfluid joins other working fluid of compressor system 100, and iscompressed by compressor 102. Compressor 102 receives the working fluidat suction 114 and compressed working fluid leaves compressor 102 atdischarge 118. The compressed working fluid then passes to condenser104. The compressor 102 may have a pressure ratio, determined by thepressure at discharge 118 divided by the pressure at suction 114.

Condenser 104 includes a heat exchanger. Condenser 104 receives thecompressed working fluid from compressor 102, and the working fluidrejects heat via the heat exchanger at condenser 104. The rejection ofheat at condenser 104 condenses the working fluid to a liquid. Condenser104 may be in thermal communication with an ambient environment, andreject heat to that ambient environment. One or more blowers 124 mayprovide airflow over the condenser 104. The one or more blowers may becontrolled by controller 110 to provide a variable airflow. In anembodiment, the one or more blowers are operated at a valuecorresponding to a percentage of the airflow that can be provided by theone or more blowers 124. The one or more blowers may affect the heattransfer of the working fluid within condenser 104. The heat transfer atcondenser 104 affects the pressure at condenser 104 and may influencethe suction pressure at suction 114 and the pressure ratio of thecompressor. Heat transfer at condenser 104, affected by the operation ofthe one or more blowers 124, also may affect the liquid line pressure atliquid line 122.

Liquid line 122 provides fluid connection between condenser 104 andexpansion device 106. One or more liquid line sensors 120 may be locatedalong liquid line 122. The one or more liquid line sensors 120 include apressure sensor to measure the pressure of the fluid within liquid line122. The pressure sensor may be any suitable pressure sensor formeasuring pressure of the fluid within liquid line 122. In anembodiment, the one or more liquid line sensors 120 further include atemperature sensor.

Cooling line 126 may provide a fluid connection between liquid line 122and components of compressor system 100 that are to be cooled. Coolingline 126 may convey some of the working fluid from liquid line 122 tothe compressor system 100 components that are to be cooled, such a motorand/or an inverter of compressor 102. The working fluid from coolingline 126 may be expanded, absorbing heat at the compressor system 100components being cooled. For the components of compressor 102, theexpansion of the working fluid from cooling line 126 may be expansion tothe intermediate pressure of the compressor 102.

Expansion device 106 is a device configured to reduce the pressure ofthe working fluid. As a result, a portion of the working fluid isconverted to a gaseous form. Expansion device 106 may be, for example,an expansion valve, orifice, or other suitable expander to reducepressure of a refrigerant such as the working fluid. In an embodiment,expansion device 106 includes multiple orifices. In an embodiment, themultiple orifices of expansion device 106 have different sizes.Expansion device 106 may be a controllable expansion device having avariable aperture. In an embodiment, expansion device 106 is anelectronic expansion valve. Expansion device 106 may be controlled bycontroller 110 to adjust its effects on flow and expansion of theworking fluid, for example by controlling aperture size of an expansionvalve or the number and size of orifices in use based on a signal fromcontroller 110.

Evaporator 108 receives working fluid from expansion device 106.Evaporator 106 includes a heat exchanger where the working fluid canabsorb heat, for example absorbing heat from an airflow to be providedto a building to cool the airflow. Compressor system 100 may be part ofan HVACR system. The HVACR system may be, for example, an air-cooledwater chiller. This airflow exchanges heat with the working fluid in theevaporator 108, which absorbs heat from the airflow and evaporates theworking fluid.

Controller 110 is a controller which may be operatively coupled to atleast suction pressure sensor 112, discharge pressure sensor 116, theone or more liquid line sensors 120, expansion device 106, and the oneor more blowers 124. Controller 110 may be operatively coupled tosuction pressure sensor 112, discharge pressure sensor 116, and the oneor more liquid line sensors 120 such that controller 110 receivespressure readings from each. Controller 110 may further be operativelyconnected to the one or more liquid line sensors 120 such thatcontroller 120 receives a liquid line temperature. Controller 110 may beoperatively coupled to expansion device 106 such that controller 110 canprovide commands to expansion device 106. Controller 110 may beoperatively connected to the one or more blowers 124 such that it canprovide commands to the one or more blowers 124. Controller 110 mayinclude one or more processors and one or more memories.

Controller 110 may be configured to determine a pressure target based onan intermediate pressure within the compressor 102 and a thresholdcooling differential pressure value. In an embodiment, the pressuretarget may be a sum of the intermediate pressure in compressor 102 andthe threshold cooling differential pressure value. In an embodiment, thethreshold cooling differential pressure value is a value stored in amemory of controller 110.

Controller 110 may be configured to determine the intermediate pressurein compressor 102. In an embodiment, controller 110 is configured todetermine the intermediate pressure based on the suction pressuremeasured by 112 and the discharge pressure measured by 116. In anembodiment, controller 110 determines the intermediate pressure incompressor 102 by taking a square root of the product of the suctionpressure measured by 112 and the discharge pressure measured by 116.

Controller 110 may be configured to determine a pressure ratio setpointbased on the pressure target and a liquid line pressure measured by theone or more liquid line sensors 120. The pressure ratio setpoint may bea desired pressure ratio. The pressure ratio may be defined by thedischarge pressure of compressor 102 divided by the suction pressure ofcompressor 102. Airflow over condenser 104 may affect the condensationof the working fluid in condenser 104 that is received from discharge118 of compressor 102 and pressure within condenser 104. The airflowover condenser 104 may thus affect the discharge pressure and pressureratio of compressor 102. In an embodiment, the pressure ratio setpointis determined by comparing the pressure target with the pressure inliquid line 122, measured by the one or more liquid line sensors 120,and when the pressure target exceeds the pressure in liquid line 122,setting the pressure ratio target to the current pressure ratiomultiplied by the ratio of the pressure target to the pressure in liquidline 122. When the pressure in liquid line 122 exceeds the pressuretarget, the pressure ratio setpoint may be maintained at a currentlevel.

Controller 110 may be configured to control the one or more blowers tooperate at a speed determined based on the determined pressure ratiosetpoint. The control of the blower may be any suitable control loopused to control airflow from the one or more blowers 124 to achieve thedetermined pressure ratio setpoint. In an embodiment, the control loopused to control the airflow is a closed-loop control. In an embodiment,the control loop used to control the airflow includes aproportional-integral-derivative (PID) control loop.

Controller 110 may be configured to determine a subcooling setpointbased on the pressure target and the liquid line pressure in thecompressor system. The subcooling setpoint is a setpoint that may beused as a control parameter for expansion device 106. Expansion device106 controls flow out of liquid line 122, and thus the subcoolingsetpoint and corresponding control of expansion device 106 may affectthe differential pressure in compressor system 100. In an embodiment,controller 110 is configured to determine the subcooling setpoint bydetermining a liquid pressure difference based on the difference betweenthe pressure target and the actual pressure within liquid line 122measured by the one or more liquid line sensors 120. The pressuredifference may be converted to a temperature offset for subcooling. Thetemperature offset may then be applied to a current subcooling value toobtain the subcooling setpoint. Controller 110 may be configured tocontrol the expansion device 106 based on the subcooling setpoint, forexample by adjusting an aperture size within expansion device 106. Thecontrol of expansion valve 106 may be through any suitable control loopfor controlling expansion device 106 based on the subcooling setpoint.In an embodiment, the control of the expansion device 106 may referencea liquid line temperature measured by the one or more liquid linesensors 120. In an embodiment, the control loop used to control theexpansion device 106 is a closed-loop control. In an embodiment, thecontrol loop used to control the expansion device 106 includes a PIDcontrol loop.

FIG. 2 shows a flowchart of a method 200 of controlling a compressorsystem according to an embodiment. A pressure target is determined 202.A liquid line pressure is received 204. A pressure ratio setpoint isdetermined 206, and a condenser blower is operated based on the pressureratio setpoint 208. Also, a subcooling setpoint is determined 210, andan expansion device is operated based on the subcooling setpoint 212.

A pressure target is determined 202. The pressure target may be apressure level to maintain cooling of components of one or morecompressor system components such as a compressor motor, an inverter, orany other component cooled at least in part using a working fluid of thecompressor system. The pressure target may be determined based on apressure differential used to provide cooling to the one or morecompressor system components. The pressure target may be based on anintermediate pressure within the compressor and a threshold coolingdifferential pressure value. In an embodiment, the pressure target isdetermined by adding a minimum compressor cooling differential pressurevalue to an intermediate pressure within the compressor. Optionally, inan embodiment the intermediate pressure is determined 218 based onmeasured suction and discharge pressures. The minimum compressor coolingdifferential pressure value may be a value stored in memory. Thethreshold cooling differential may be determined based on the coolingdemand of components of the compressor system. In an embodiment, thethreshold cooling differential may be a value calculated based on one ormore operational parameters of the compressor. In an embodiment, thethreshold cooling differential may be determined based on a mathematicalmodel correlating pressure required for cooling with the one or moreoperational parameters of the compressor.

A liquid line pressure is received 204. The liquid line pressure may bea value measured by a pressure sensor located along the liquid line,such as the one or more liquid line sensors 120 along liquid line 122 asshown in FIG. 1 and described above.

A pressure ratio setpoint is determined 206. The pressure ratio setpointmay be based on the pressure target determined at 202 and the liquidline pressure received at 204. The pressure ratio may be defined by thedischarge pressure of the compressor of the compressor system divided bythe suction pressure of the compressor. Airflow over a condenser of thecompressor system may affect the discharge pressure, and thus thepressure ratio. The pressure ratio setpoint thus may be used to controlairflow over the condenser and the pressure at compressor discharge andthe liquid line pressure. In turn, this affects the differential betweenliquid line pressure and the intermediate pressure and thus the coolingprovided to components of the compressor system. In an embodiment, thepressure ratio setpoint may be determined by comparing the pressuretarget and the liquid line target and based on that comparison,determining the pressure ratio setpoint, as shown in FIG. 3 anddescribed below.

A condenser blower is operated based on the pressure ratio setpoint 208.The operation of the condenser blower based on the pressure ratiosetpoint 208 may be accomplished through a control loop for condenserblower airflow including pressure ratio setpoint as a variable. In anembodiment, a mathematical model correlating pressure ratio setpointwith airflow may be referenced to operate the condenser blower accordingto the pressure ratio setpoint 208. In an embodiment, the speed ofrotation of the condenser blower is adjusted according to the controlloop. In an embodiment, the control loop used to control the airflow isa closed-loop control. In an embodiment, the control loop used tocontrol the airflow includes a proportional-integral-derivative (PID)control loop.

A subcooling setpoint is determined 210. The subcooling setpoint may bea value for subcooling used in control of an expansion device of thecompressor system. The expansion device may control flow out of a liquidline of the compressor system, and thus the expansion device may controlthe liquid line pressure for the compressor system. The liquid linepressure may affect whether one or more compressor system components maybe cooled by a cooling system, for example when such cooling systemsrely on a pressure difference between the liquid line and anintermediate pressure within the compressor. The subcooling setpoint maybe based on a subcooling value and an offset, the offset computed basedon the pressure target determined at 202 and the liquid line pressurereceived at 204. In an embodiment, the subcooling setpoint may bedetermined by determining a liquid pressure difference, determining asubcooling offset, and adding the subcooling offset to a currentsubcooling value as shown in FIG. 4 and described below.

An expansion device is operated based on the subcooling setpoint 212.The operation of the expansion device according to the subcoolingsetpoint 212 may be accomplished through a control loop includingsubcooling setpoint as a variable. The expansion device may be, forexample, expansion device 106 shown in FIG. 1 and described above. Theexpansion device may have a controllable aperture size. In anembodiment, the expansion device is an electronic expansion valve. In anembodiment, an aperture size of the expansion device is controlled basedon the subcooling setpoint 212. In an embodiment, a mathematical modelcorrelating expansion device position with subcooling setpoint isreferenced to operate the expansion device. Control based on thesubcooling setpoint may reference a current liquid line temperature. Inan embodiment, the liquid line temperature may be measured by atemperature sensor. In an embodiment, the control loop used to controlthe expansion device is a closed-loop control. In an embodiment, thecontrol loop used to control the expansion device includes a PID controlloop.

Optionally, method 200 may include receiving the suction pressure 214,receiving the discharge pressure 216, and/or determining theintermediate pressure in the compressor 218. In an embodiment, thesuction pressure may be received from a pressure sensor at a suctionport of the compressor, such as suction pressure sensor 112 shown inFIG. 1 and described above. In an embodiment, the discharge pressure maybe received from a pressure sensor at the discharge port of thecompressor, such as discharge pressure sensor 116 shown in FIG. 1 anddescribed above. In an embodiment, the intermediate pressure isdetermined 218 based on one or more of the suction pressure received at214 and the discharge pressure received at 216. In an embodiment, theintermediate pressure is determined 218 by taking the square root of theproduct of the suction pressure received at 214 and the dischargepressure received at 216. In an embodiment, the intermediate pressuredetermined at 218 is used to determine the pressure target 202.

During operation, the method 200 may iterate, for example by returningfrom operating the condenser blower 208 and operating the expansiondevice 212 to receiving the suction pressure 214, receiving thedischarge pressure 216, or determining the pressure target 202. Theiteration may be continuous, periodic, or triggered based on conditionssuch as changes to the operating profile of the compressor system,temperature in components of the compressor system, or ambientenvironmental conditions.

FIG. 3 shows a flowchart of an example method for determining a pressureratio setpoint according to an embodiment. In the embodiment shown inFIG. 3 , when the liquid line pressure is received 204, the pressuretarget determined at 202 and the liquid line pressure received at 204are compared 302.

When the pressure target determined at 202 is found to exceed liquidline pressure received at 204 in the comparison at 302, the pressureratio setpoint is determined based on the current pressure ratiomultiplied by a ratio of the pressure target to the liquid line pressure304.

In an embodiment, the formula for determining the pressure ratio targetis:

${{Pressure}{Ratio}{Setpoint}} = {\left( \frac{{Pressure}{Target}}{{Liquid}{Line}{Pressure}} \right) \star \left( \frac{{Discharge}{Pressure}}{{Suction}{Pressure}} \right)}$

When the liquid line pressure received at 204 is found to exceed thepressure target determined at 202, the pressure ratio setpoint may bemaintained at the current pressure ratio 306.

Once the pressure ratio setpoint is established at either 304 or 306,based on the comparison at 302, the condenser blower may be controlled208 based on the pressure ratio setpoint 204, as described above.

FIG. 4 shows a flowchart of an example method for determining asubcooling setpoint according to an embodiment. After the targetpressure is determined at 202 and the liquid line pressure is receivedat 204, a liquid pressure difference is determined 402, a subcoolingoffset is determined 404, and the subcooling offset is added to acurrent subcooling value 406.

A liquid pressure difference is determined 402. The liquid pressuredifference may a value in units of pressure, for example kPa. The liquidpressure difference may be a difference between the target pressuredetermined at 202 and the liquid line pressure received at 204. In anembodiment, the liquid pressure difference is determined 402 bysubtracting the liquid line pressure received at 204 from the targetpressure determined at 202.

A subcooling offset is determined 404 based on the liquid pressuredifference determined at 402. The subcooling offset may be a temperaturevalue having units in ° F., ° C., or K. In an embodiment, a function isused to convert the liquid pressure difference determined at 402 into asubcooling offset. In an embodiment, a mathematical model correlatingliquid pressure difference and subcooling offset is referenced. In anembodiment, a lookup table correlating liquid pressure difference andsubcooling offset is referenced. In an embodiment, the subcooling offsetdetermined at 404 is proportional to the liquid pressure differencedetermined at 402.

The subcooling offset is added to a current subcooling value 406. Thesubcooling setpoint determined by adding the subcooling offset to thecurrent subcooling value at 406 may then be used to control an expansiondevice 212 as described above.

Aspects:

It is understood that any of aspects 1-10 can be combined with any ofaspects 11-16.

Aspect 1. A method of controlling a compressor system, comprising:

determining a pressure target based on an intermediate pressure within acompressor of the compressor system and a threshold cooling differentialpressure value;

determining a pressure ratio setpoint based on the pressure target and aliquid line pressure within a liquid line of the compressor system;

operating an condenser blower at a speed, the speed determined based onthe determined pressure ratio setpoint;

determining an subcooling setpoint based on the pressure target and theliquid line pressure in the compressor system; and

operating an expansion device based on the determined subcoolingsetpoint.

Aspect 2. The method according to aspect 1, wherein the intermediatepressure is determined based on a suction pressure of the compressor anda discharge pressure of the compressor.

Aspect 3. The method according to any of aspects 1-2, whereindetermining the pressure ratio setpoint comprises:

comparing the pressure target and the liquid line pressure;

when the pressure target exceeds the liquid line pressure, setting thepressure ratio setpoint to a current pressure ratio multiplied by aratio of the pressure target to the liquid line pressure;

when the liquid line pressure meets or exceeds the pressure target,setting the pressure ratio setpoint to the current pressure ratio.

Aspect 4. The method according to aspect 3, wherein determining thespeed comprises referencing a mathematical model correlating pressureratio and condenser blower speed.

Aspect 5. The method according to any of aspects 3-4, further comprisingapplying a smoothing function to control a rate of change of thepressure ratio setpoint.

Aspect 6. The method according to any of aspects 3-5, further comprisingcomparing the determined pressure ratio setpoint to a pressure ratiolimit and setting the pressure ratio setpoint to the pressure ratiolimit when the determined pressure ratio setpoint exceeds the pressureratio limit.

Aspect 7. The method according to any of aspects 1-6, whereindetermining the subcooling setpoint comprises:

determining a liquid pressure difference based on the pressure targetand the liquid line pressure;

determining a subcooling offset based on the liquid pressure difference;

determining the subcooling setpoint based on the subcooling offset and acurrent subcooling value.

Aspect 8. The method according to aspect 7, wherein determining thesubcooling offset comprises referencing a mathematical model correlatingthe subcooling offset with the liquid pressure difference.

Aspect 9. The method according to any of aspects 7-8, further comprisingapplying a smoothing function to control a rate of change in thesubcooling setpoint.

Aspect 10. The method according to any of aspects 7-9, wherein operatingthe expansion device based on the subcooling setpoint comprises settingan aperture size of the expansion device based on the subcoolingsetpoint.

Aspect 11. A compressor system, comprising:

a compressor;

a condenser;

an expansion device fluidly connected to the condenser by a liquid line;

a liquid line pressure sensor at a liquid line of the compressor system;

an evaporator;

a blower configured to drive an airflow over the condenser; and

a controller, configured to:

determine a pressure target based on an intermediate pressure within thecompressor and a threshold cooling differential pressure value;

determine a pressure ratio setpoint based on the pressure target and aliquid line pressure measured by the liquid line pressure sensor;

control the blower to operate at a speed determined based on thedetermined pressure ratio setpoint;

determine a subcooling setpoint based on the pressure target and theliquid line pressure in the compressor system; and control the expansiondevice to operate based on the subcooling setpoint.

Aspect 12. The compressor system according to aspect 11, wherein theexpansion device is a controllable electronic expansion valve.

Aspect 13. The compressor system according to aspect 12, wherein theprocessor is configured to control the expansion device to operate basedon the subcooling setpoint by setting an aperture size for thecontrollable electronic expansion valve.

Aspect 14. The compressor system according to any of aspects 11-13,further comprising a suction pressure sensor located at the suction ofthe compressor and a discharge pressure sensor located at the dischargeof the compressor, and wherein the processor is further configured todetermine the intermediate pressure within the compressor based on asuction pressure measured by the suction pressure sensor and a dischargepressure measured by the discharge pressure sensor.

Aspect 15. The compressor system according to any of aspects 11-14,wherein the controller is configured to determine a pressure ratiosetpoint by:

comparing the pressure target and the liquid line pressure;

when the pressure target exceeds the liquid line pressure, setting thepressure ratio setpoint to a current pressure ratio multiplied by aratio of the pressure target to the liquid line pressure; and

when the liquid line pressure meets or exceeds the pressure target,setting the pressure ratio setpoint to the current pressure ratio.

Aspect 16. The compressor system according to any of aspects 11-15,wherein the controller is configured to determine a subcooling setpointby:

determining a liquid pressure difference based on the pressure targetand the liquid line pressure;

determining a subcooling offset based on the liquid pressure difference;and

determining the subcooling setpoint based on the subcooling offset and acurrent subcooling value.

The examples disclosed in this application are to be considered in allrespects as illustrative and not limitative. The scope of the inventionis indicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

The invention claimed is:
 1. A method of controlling a compressorsystem, comprising: determining a pressure target based on a pressuredifferential used to provide cooling to one or more compressor systemcomponents; determining a pressure ratio setpoint based on the pressuretarget and a liquid line pressure within a liquid line of the compressorsystem; operating a condenser blower at a speed, the speed determinedbased on the determined pressure ratio setpoint; determining asubcooling setpoint based on the pressure target and the liquid linepressure in the compressor system; and operating an expansion devicebased on the determined subcooling setpoint.
 2. The method of claim 1,wherein determining the pressure ratio setpoint comprises: comparing thepressure target and the liquid line pressure; when the pressure targetexceeds the liquid line pressure, setting the pressure ratio setpoint toa current pressure ratio multiplied by a ratio of the pressure target tothe liquid line pressure; when the liquid line pressure meets or exceedsthe pressure target, setting the pressure ratio setpoint to the currentpressure ratio.
 3. The method of claim 2, wherein determining the speedcomprises referencing a mathematical model correlating pressure ratioand condenser blower speed.
 4. The method of claim 2, further comprisingapplying a smoothing function to control a rate of change of thepressure ratio setpoint.
 5. The method of claim 2, further comprisingcomparing the determined pressure ratio setpoint to a pressure ratiolimit and setting the pressure ratio setpoint to the pressure ratiolimit when the determined pressure ratio setpoint exceeds the pressureratio limit.
 6. The method of claim 1, wherein determining thesubcooling setpoint comprises: determining a liquid pressure differencebased on the pressure target and the liquid line pressure; determining asubcooling offset based on the liquid pressure difference; determiningthe subcooling setpoint based on the subcooling offset and a currentsubcooling value.
 7. The method of claim 6, wherein determining thesubcooling offset comprises referencing a mathematical model correlatingthe subcooling offset with the liquid pressure difference.
 8. The methodof claim 6, further comprising applying a smoothing function to controla rate of change in the subcooling setpoint.
 9. The method of claim 6,wherein operating the expansion device based on the subcooling setpointcomprises setting an aperture size of the expansion device based on thesubcooling setpoint.
 10. The method of claim 1, wherein the determiningof the pressure target is based on a threshold cooling differential. 11.The method of claim 10, wherein the threshold cooling differential isbased on one or more operational parameters of the compressor.
 12. Themethod of claim 1, wherein the determining of the pressure target isbased on an intermediate pressure within the compressor.
 13. Acompressor system, comprising: a compressor; a condenser; an expansiondevice fluidly connected to the condenser by a liquid line; a liquidline pressure sensor at a liquid line of the compressor system; a blowerconfigured to drive an airflow over the condenser; and a controller,configured to: determine a pressure target based on a pressuredifferential used to provide cooling to one or more compressor systemcomponents; determine a pressure ratio setpoint based on the pressuretarget and a liquid line pressure measured by the liquid line pressuresensor; control the blower to operate at a speed determined based on thedetermined pressure ratio setpoint; determine a subcooling setpointbased on the pressure target and the liquid line pressure in thecompressor system; and control the expansion device to operate based onthe subcooling setpoint.
 14. The compressor system of claim 13, whereinthe expansion device is a controllable electronic expansion valve. 15.The compressor system of claim 14, wherein the processor is configuredto control the expansion device to operate based on the subcoolingsetpoint by setting an aperture size for the controllable electronicexpansion valve.
 16. The compressor system of claim 13, wherein thecontroller is configured to determine a pressure ratio setpoint by:comparing the pressure target and the liquid line pressure; when thepressure target exceeds the liquid line pressure, setting the pressureratio setpoint to a current pressure ratio multiplied by a ratio of thepressure target to the liquid line pressure; and when the liquid linepressure meets or exceeds the pressure target, setting the pressureratio setpoint to the current pressure ratio.
 17. The compressor systemof claim 13, wherein the controller is configured to determine asubcooling setpoint by: determining a liquid pressure difference basedon the pressure target and the liquid line pressure; determining asubcooling offset based on the liquid pressure difference; anddetermining the subcooling setpoint based on the subcooling offset and acurrent subcooling value.
 18. The compressor system of claim 13, whereinthe controller is configured to determine the pressure target based on athreshold cooling differential.
 19. The compressor system of claim 18,wherein the threshold cooling differential is based on one or moreoperational parameters of the compressor.
 20. The compressor system ofclaim 13, wherein the controller is configured to determine the pressuretarget based on an intermediate pressure within the compressor.